The present invention relates to a coding system of a document image, and in particular, to a coding system for improving a data compression efficiency in an operation to code a pseudo halftone picture which is a continuous-tone picture represented with binary values associated with black and white.
Conventionally, there has been discussed a high-efficiency coding of a binary picture or image to be primarily employed in a high-speed facsimile. As for the highly efficient coding methods of the facsimile, there have been broadly adopted the modified Huffmann (MH) coding system and the modified relative element address designate (MR) coding system stipulated according to the international standards with respect to the compatibility and the communicability. These coding systems are widely utilized in the facsimile communication as well as other operations such as filing of document images.
The MH coding system is a one-dimensional coding system in which run lengths of black and white are subjected to a variable length coding by paying attention to a scanning line so as to effect a one-dimensional coding. The run length here indicates the length of the black or white pixels successively arranged in the scanning line direction. The MH coding system is a twodimensional coding system in which the coding is achieved by paying attention to a contour of a boundary line with respect to black and white between a scanning line to be coded and a reference scanning line immediately thereabove.
These coding systems have been stipulated on assumption of applications of the ordinary document images and drawings so as to establish matching with the static characteristics thereof. However, there exist many document images or pictures which do not conform thereto. For example, these images include a screened photograph having a fine periodic pattern therein and a dithed image with a characteristic periodic pattern generated by a computer. When the MH or MR coding system is applied to these images, the coding efficiency is lowered, and in an extreme case, the number of code bits thus obtained exceeds that of the pixels.
In order to avoid such a disadvantage, there exists an adaptive coding/direct change-over system in which the coding is not effected in a region of a fine image so as to directly transmit a series of the pixels. The prior art related to this system has been described, for example, in the JP-A No. 50-3713 "A Coding Method for Binary Signals", Fukinuki. The uncompressed mode in the international standards of the facsimile is also one of the coding systems. Details thereabout has been stipulated in the Recommendation T.6 of the Comite Consulatif International Telegraphique et Telephinique (CCITT).
When the operation is switched to the uncompressed mode, there is inserted an identification code word (0000001111, ten bits). When the uncompressed mode is released, there is inserted another identification code word (0000001T; eight bits, T=Additional bit indicating a color of the run, namely, 1 for black and 0 for white). In consequence, if there frequently occurs a change-over between the compressed mode and the uncompressed mode, the total amount of codes (the number of code bits attained by the coding operation) is undesirably increased. To overcome this difficulty, there has been proposed a mode change-over judging method which does not lead to an increase of the amount of the codes.
One of such methods is a code length monitor method or code word detecting method in which each code length is monitored for judgment of the mode change-over (PROC, ANN, Meeting of IECE (in Japan)). In this method, the judgment is conducted through the following procedure.
(1) In a case where the MH (MR) coding is achieved (in the compressed mode), if the next run length is great and the MH (MR) code includes bits less than the code length thereof in the uncompressed mode, the MH (MR) code system is adopted for the coding operation.
(2) In contrary, when the MH (MR) code includes bits greater than the code length thereof in the uncompressed mode, the coding operation is reserved and the difference between the code lengths of the respective modes is consecutively accumulated. Only when the difference exceeds the total (18 bits) of the identification code words, the operation is changed over to the uncompressed mode.
(3) After the operation is changed over to the uncompressed mode, the judgment is conducted by replacing the compressed mode with the uncompressed mode.
However, when it is desired to implement the code length monitor method in which the code length is monitored for the judgment of the mode change-over, there arises a long reservation period of time associated with the coding operation as described in Article 2 above; in consequence, there appears a situation where the coding operation cannot be effected for a long period of time. In the worst case, the coding mode cannot be determined even at a stage where a page of document image has been coded. In consequence, the processing time required for the coding operation is increased and hence a large capacity is necessary for the buffer memory to store the coded data associated with both modes.
There exists another mode judge method in which the compressed and uncompressed modes are judged depending on a characteristic of a dithered image. There has been proposed a method in which the run length is employed as the characteristic of the image (PROC, ANN, Meeting of IECE (in Japan)).
In this method, when a combination of the run lengths 1 and 1 or 1 and 3 continuously appears several times, the operation transition takes place to the uncompressed mode. For example, the consecutive appearance count is set to two as the reference for the judgment.
Furthermore, when the number of detections of a long run exceeds a predetermined number, the operation transition takes place to the compressed mode. For example, the reference for the judgment is set to a point of time when a run having a run length of eight appears once or when a run having a run length equal to or more than four appears at least twice.
According to the judging method conducted through a characteristic detection utilizing the characteristic of the dithered image, there does not occur the problem associated with the code length monitor method, namely, the problem of the long coding processing time and the large buffer memory capacity. However, in this judge method conducted through a characteristic detection utilizing the characteristic of the dithered image, the kinds of objective dither operations are restricted. In this situation, the dither method in which there occur many combinations of the run length 1 and 1 or 1 and 3 corresponds to a case of a dithered pattern of the Bayer type associated with an organized dither method of distributed type. In consequence, in a case of a dithered pattern of a vortex type associated with an organized dither method of concentrated type or the clustered-dot ordered dither method, the mode judgment cannot be accomplished in this method. That is, there exists a disadvantage that the objective dither method is limited.